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USGS CMG InfoBank: Cross-Beds Determines Direction

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Comment: 15:20 - 16:08 (00:48)
Source: Annenberg/CPB Resources - Earth Revealed - 17. Sedimentary Rocks: The Key to Past Environments
Keywords: "Walter E. Reed", "sedimentary rock", "clastic sediment", "sedimentary structure", cross-bedding, transportation, river, dune, "water current", downstream, meander, sorting, "grain size"

Our transcription: Geologists find cross-beds useful in determining the direction of sediment transport in ancient river and dune systems.
Cross-beds form perpendicular to the direction of the water current and tend to slope downstream.
If you've ever looked at a river in any kind of detail, you know that a river tends not to run in a straight line; it meanders around.
And so if we get many, many cross-bed measurements, then we can come up with a statistical average for the direction the river was flowing.
If we go downstream, down that river we'll find that sorting becomes better, sediment size tends to become less, and so forth.

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